The discriminant b^2-4ac is the part of the quadratic formula under the square root sign so if b^-4ac is negative then part of the root will contain the square root of a negative number. There is no real number which satisfies this so the roots are imaginary
In this case b^2 -4ac = (2)^2 - (4x5x1)= 4-20 = -16
Since the discriminant is negative the roots are therefore imaginary